The present invention relates to a large capacity apparatus for generating a high purity acid or base particularly for use as a chromatography eluent, and to a method of using the apparatus.
In liquid chromatography, a sample containing a number of components to be separated is directed through a chromatography separator, typically an ion exchange resin bed. The components are separated on elution from the bed in a solution of eluent. One effective form of liquid chromatography is referred to as ion chromatography. In this known technique, ions to be detected in a sample solution are directed through the separator using an eluent containing an acid or base and thereafter to a suppressor, followed by detection, typically by an electrical conductivity detector. In the suppressor, the electrical conductivity of the electrolyte is suppressed but not that of the separated ions so the latter may be detected by the conductivity detector. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,956,559.
There is a general need for a convenient source of high purity acid or base for use as an eluent for liquid chromatography and, particularly, for ion chromatography. In one technique, described in U.S. Pat. No. 5,045,204, an impure acid or base is purified in an eluent generator while flowing through a source channel along a permselective ion exchange membrane which separates the source channel from a product channel. The membrane allows selective passage of cations or anions. An electrical potential is applied between the source channel and the product channel so that the anions or cations of the acid or base pass from the former to the latter to generate therein a base or acid with electrolytically generated hydroxide ions or hydronium ions, respectively. This system requires an aqueous stream of acid or base as a starting source or reservoir.
There is a particular need for a pure source of acid or base which can be generated at selected concentrations solely from an ion exchange bed without the necessity of an independent reservoir of an acid or base starting aqueous stream. There is a further need for such a system which can be continuously regenerated. Such need exists in chromatography, and specifically ion chromatography, as well as other analytical applications using acid or base such as in titration, flow injection analysis and the like.
In copending application Ser. No. 08/783,317, filed Jan. 15, 1997, a method and apparatus is described for generating acid or base in an aqueous stream, such as water alone or in combination with additives (e.g., ones which react with the acid or base or with the sample). The system provides an excellent source of high purity acid or base for use as an eluent for chromatography and, particularly, ion chromatography. The present system is an improvement over the one described in the copending application.
Referring first to the present system in which a base is generated e.g. for chromatographic analysis of anions, the method includes the steps of:
(a) providing a cation source in a cation source reservoir,
(b) flowing an aqueous liquid stream through a base generation chamber separated from the cation source reservoir by a barrier substantially preventing liquid flow while providing a cation transport bridge,
(c) applying an electric potential between an anode in electrical communication with said cation source reservoir and a cathode in electrical communication with the base generation chamber to electrolytically generate hydroxide ions in the base generation chamber and to cause cations in the cation source reservoir to electromigrate toward said first barrier and to be transported across the barrier toward the cathode to combine with the transported cations to form cation hydroxide, and
(d) removing the cation hydroxide in an aqueous liquid stream as an effluent from the first base generation chamber.
Suitable cation sources include a salt solution or a cation hydroxide solution which can be supplied to the cation source reservoir by pumping from a remote reservoir. The solution can be recycled to the remote reservoir. Also, the cation source may comprise a cation exchange bed, e.g., resin particles in a stationary bed or suspended in an aqueous liquid, alone or in combination with the salt solution.
The method may also be used for generating an acid, e.g. for use as an eluent for chromatographic analysis of cations by reversing the charges of the ion source, the barrier, the electrical potential and any other charged components of the system.
Another embodiment of the invention is an apparatus for generating an acid or base including:
(a) an ion source reservoir of either anions or cations,
(b) an acid or base generation chamber having inlet and outlet ports,
(c) a first barrier between the ion source reservoir and the acid or base generation chamber, substantially preventing liquid flow while providing an ion transport bridge for only ions of one charge, positive or negative,
(d) a first electrode in electrical communication with the ion source reservoir,
(e) a second electrode in electrical communication with the first acid or base generation chamber, and
(f) an aqueous liquid source in fluid communication with the acid or base generation chamber inlet port.
The apparatus can be used to supply the generated acid or base to a chromatography system or any other analytical system which uses a high purity acid or base.